Water softeners and leaking valve to drain or constant regeneration
Listen to the control valve on top of the resin tanks.

Reverse Osmoses (R.O.) under kitchen sink and constant flows into sink drain
Listen under the sink for constant drainage. R.O. wastes approximately 2-5 gallons of water to make each gallon of the R.O. drinking water, but it makes the water slowly. So if you took a glass of water out of the drinking faucet, it may take 30 minutes to remake that glass you took out of the R.O. storage tank.

Water troughs and overflowing
Turn them off until you replace the floats.

Hose bibs (Especially those coming from the ground)
Listen to the hose bib. Many can have small leaks underground that percolate
Into the soil and never come to the surface.

Drip systems connected to hose bibs (Many are leaking and go unnoticed)
Disconnect from the hose bib to visually make sure the hose bib is not leaking (turned off fully)
If on an automatic battery controller, make sure the controller is set properly and not on manually or programmed to come on for too long or too often.
Many hose bibs are turned on and you forget to turn them back off. I use spring loaded manual twist on timers that can be set up to 2 hours. Orbit makes them for Amazon for about $12 each.

Irrigation valves leaking and not positively shutting off
Listen to each valve in the valve boxes (watch out for Black Widow Spiders)
These valves that have slow leaks will drain water to the lowest head on that valve station and slowly run out.

Auto fill valves on pools, ponds, fountains and leaking floats
Turn off the auto fill valves for a couple of days or listen to the valve.

An automotive stethoscope aids tremendously to help hear sources of leaks described in the above hints. Amazon has one for under $10. The link is below---
https://www.amazon.com/s?k=automotive+stethoscope&crid=1JJB0L9KG1G64&sprefix=automotive%2Caps%2C223&ref=nb_sb_ss_ts-doa-p_1_10